AMD Performance Rating

A Performance Rating system is an alternative way of assessing CPU speed,
instead of just looking at clockspeed.

Intel and AMD have adhered to pure MHz ratings until recently.
The way an AMD CPU is designed it generally runs at lower clockspeeds than its Intel
counterpart, while maintaining similar performance. This is because AMD claims their
instructions per cycle (IPC) is higher, meaning that per MHz an AMD CPU ‘does more.’
While that may not be a hard concept the grasp, not everyone is aware of it.
AMD revived the PR in an attempt to make the CPUs total performance more apparent.
Their claim is that the PR rating on a chip denotes how fast an AMD Athlon ‘Thunderbird”
processor would have to be to match the rated chip.
from: http://www.ocfaq.com/article.php/overclocking/AMD/36